In ink-jet printing, minute droplets of black or coloured ink are ejected in a controlled manner from one or more reservoirs or printing heads through narrow nozzles on to a substrate, which is moving relative to the reservoirs. The ejected ink forms an image on the substrate. For high-speed printing, the inks must flow rapidly from the printing heads, and to ensure that this happens they must have a low viscosity at the jetting stage. The ink viscosity is typically below 25 mPas at jetting temperature.
One type of ink contains unsaturated organic monomers or oligomers that polymerise by irradiation, commonly with UV light, in the presence of a photoinitiator. This type of ink has the advantage that it is not necessary to evaporate the liquid phase to dry the print; instead the print is exposed to radiation to cure or harden it, a process that is more rapid than evaporation of solvent at moderate temperatures.
There are two main technologies that can be used in a UV curing processes. The first method uses free-radical species to initiate the polymerisation of reactive monomers such as acrylate or methacrylate esters, and the second involves the generation of very strong acids to initiate the cationic polymerisation of reactive monomers such as epoxides, allyl ethers and vinyl ethers.
In UV printers the ink is cured shortly after printing by exposure to an intense UV light source. The ink, more particularly the photoinitiator provided in the ink, is generally tailored to respond to radiation having the particular wavelength(s) emitted by the UV source, principally to prevent curing in ambient light. The source should therefore have sufficient output intensity in this spectral region.
The most common UV light source used to cure printer ink is the mercury discharge lamp. These lamps operate by creating a plasma between two electrodes in a high pressure mercury gas contained in a quartz envelope. Although these lamps have several drawbacks in their operational characteristics, no other UV light source has yet managed to challenge their position in terms of UV output performance.
Since mercury is a liquid at room temperature, mercury discharge lamps must heat up, to typically 800° C., to establish a stable, high pressure mercury gas that gives a consistent output. Depending on the lamp size, the lamp must therefore be switched on for between 2-30 minutes before it can be used, which impacts on printing productivity.
The mercury spectrum has strong emission lines (peaks) at 254, 365 (I-line), 405 (H-line), 436 (G-line), 546, and 578 nm, though these can be influenced by the purposeful inclusion of impurities in the lamp. Many commercially available photoinitiators have been developed that respond to these peak emission wavelengths, making the development of UV curable inks that respond to mercury discharge lamps commercially effective. Unfortunately, the 254 nm emission is responsible for the efficient production of ozone in air, which can have consequences on human health if not properly managed. The mercury contained in the lamp also poses a threat to health if not properly managed throughout its full lifetime, which includes environmental disposal.
By adjusting the current flow through a mercury discharge lamp, the output power can be controlled, thereby affecting the flow characteristics of the ink on the receiving substrate. Exposing ink droplets to a high lamp power will cause the droplets to cure before they can spread and merge. This leads to a highly roughened surface with diffuse reflectance that gives a matt image effect, sometimes referred to as “satin”. Conversely, low incident UV powers will allow the ink to flow slightly before curing is achieved, reducing the finished print surface roughness so that a “gloss” effect is achieved. A minimum current must be maintained to prevent the mercury discharge lamp plasma extinguishing. The output power range is therefore typically adjustable only between 25-100%. A proportional shutter mechanism is therefore required to control ink response to output powers below 25%, adding material cost to printers that can print both gloss and satin finish images.
The output power intensity of a mercury discharge lamp can fall rapidly with use, meaning that the lamps are considered a consumable item within printers, that have to be replaced approximately every 4-6 months. However, for stability of output to maintain the same degree of print finish (matt/satin/gloss), the intensity of the lamp must be routinely measured and the supplied power adjusted to compensate for reductions in lamp efficiency. These processes can be time-consuming and will have a detrimental impact on printer productivity.
LED (light emitting diode) UV light sources are an attractive alternative to mercury lamps. As well as their lack of toxicity, LEDs have several advantages over mercury discharge lamps. The output of the LED is immediate without a warm-up period and can be controlled from 0-100% by adjusting the device driving current. This relaxes the requirement for expensive drive electronics and shutters when used in printers. LEDs are also robust with lifetimes in excess of 5 years, which means that print finish reproducibility would be high without the necessity for repeated output power recalibration. LEDs also have a narrow spectral output and so the radiant energy produced is largely free from infrared, which has led LEDs to being heralded as a cool UV source.
Most graphical inks that have been developed for use with mercury discharge lamps have their peak response to light at about 365 nm. Unfortunately, as the LED emission wavelength is reduced below ˜450 nm, the output efficiency falls rapidly. Any input power not converted to light is efficiently converted to heat. Furthermore, the UV output power of current LED sources is still relatively low when compared to mercury lamps. LED sources therefore have to be driven hard in order to achieve the UV outputs that are required to fully cure currently available inks. This results in heat being generated, which has to be removed to prevent the LEDs failure due to overheating. Cooling equipment adds considerably to the complexity and the cost of the printer design.
One solution is to run the LED curing units at lower output power. This, however, leads to poor curing at the surface of inks that cure by radical polymerisation, due to the presence of oxygen in the atmosphere adjacent to the ink surface. This effect can be overcome by blanketing the irradiated area with an inert gas such as nitrogen during the cure process but, again, this adds considerably to the complexity and cost of the printer. There is also a danger of asphyxiation if the nitrogen builds up in the vicinity of the print machine.
Since the LEDs are not made specifically for the UV-curing industry, they remain relatively expensive when compared to high volume production LEDs made for the lighting industry. For this reason, the LEDs currently comprise about 60% of the material costs of a UV-LED cure head. The high number of LEDs that would be needed for a static cure-bar to cover a 1.6 m width print makes this method of achieving full ink cure prohibitively expensive.
There is therefore a need for a UV printing method that overcomes the above disadvantages.